Note: Descriptions are shown in the official language in which they were submitted.
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TITLE: METHOD OF GENERATING A BLOCKINESS INDICATOR FOR A
VIDEO SIGNAL
Technical Field
[0001] The described embodiments relate to methods and systems for
detecting the blockiness of a video signal.
Background
[0001] Video compression is the process of taking a video signal and
compressing it so that it will take up less bandwidth when it is transmitted
or
less space when it is stored. Many video compression standards have been
developed and include H.262, H.263, MPEG-1, MPEG-2, and MPEG-4. Many
of these standards separate video into blocks of data and separately
compress each block using a Discrete Cosine Transform (DCT). One
problem with these compression algorithms, however, is that video that
undergoes such compression may be subject to "blockiness", meaning that
the block and macroblock structure of the picture is visible.
[0002] A blockiness measurement is a quantification of the blockiness
of the decompressed video image. The blockiness measurement can be
used to determine the quality of the compressed video signal since blockiness
is a severe compression artifact. The blockiness artifact is especially
apparent at low bit rates and when the compressed signal has been
corrupted. At low video bit rates over-compression would cause visible block
transitions in smooth textual areas, such as sky, grass, or the shade of a
wall.
At low or high video bit rates, corruption of a compressed signal would cause
visible foreign blocks in the picture.
[0003] Ideally the amount of compression is maximized while still
providing a video image or picture that is free of artifacts such as
blockiness.
Accordingly there is a need for a real-time blockiness detection system which
can be used to decrease the level of compression if the blockiness reaches a
certain threshold or increase the level of compression if the blockiness falls
below a certain threshold.
Summary
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[0004] Certain embodiments relate to a method of measuring the
blockiness of a video signal comprised of a plurality of pixels, the method
comprising the steps of: calculating a total number of pixels in flat blocks
and
a total number of pixels in visible block edge transitions in the video
signal;
and generating at least one blockiness indicator from the total number of
pixels in flat blocks and the total number of pixels in visible block edge
transitions.
[0005] The method may also include the step of selecting a region of
interest from the video signal prior to calculating and generating.
[0006] The step of calculating the total number of pixels in flat blocks
areas and the total number of pixels in visible block edge transitions may
include for each pixel in the video signal: calculating differential values
for the
pixel; and analyzing the differential values to determine if the pixel is part
of a
transition and if the pixel is part of a flat area; and counting the number of
pixels in flat blocks and the number of pixels in visible block edge
transitions
to produce a total number of pixels in flat blocks and a total number of
pixels
in visible block edge transitions.
[0007] Other embodiments relate to a video signal compression system
comprising: a video source for producing a video signal comprised of a
plurality of pixels; a compression module for compressing the video signal at
a
compression level into a compressed video signal; a decompression module
for decompressing the compressed video signal into a decompressed video
signal; and a blockiness detection module configured to: calculate a total
number of pixels in flat blocks and a total number of pixels in visible block
edge transitions in the decompressed video signal; and generate at least one
blockiness indicator from the total number of pixels in flat blocks and the
total
number of pixels in visible block edge transitions wherein the compression
module adjusts the compression level based on the at least one blockiness
indicator.
Brief Description of the Drawings
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[0008] For a
better understanding of the embodiments and to show
more clearly how they may be carried into effect, reference will now be made,
by way of example only, to the accompanying drawings, in which:
[0009]
Figure 1 is a general block diagram of a blockiness detection
system in accordance with an embodiment;
[0010]
Figure 2 is a method for detecting the blockiness of a video
signal in accordance with an embodiment;
[0011]
Figure 3 is a region of interest in accordance with an
embodiment;
[0012] Figure 4 is a 3x3 luma filter kernel in accordance with an
embodiment; and
[0013]
Figure 5 is a general block diagram of a blockiness detection
system in accordance with an alternative embodiment.
Detailed Description of the Embodiments
[0014] Referring now to Figure 1, shown therein is a block diagram of a
blockiness detection system 10 including a video source 12, a compression
module 14, a decompression module 16, a blockiness detection module 18
and an optional display module 28. The video source 12 produces a video
signal 20. Compression module 14 compresses the video signal 20 using any
known compression standard such as H.261, H.263, MPEG-1, MPEG-2, and
MPEG-4 and outputs a compressed video signal 22. Decompression module
16 decompresses the compressed video signal 22 and outputs a
decompressed video signal 24. The blockiness detection module 18 analyzes
the decompressed video signal 24 and generates a decompressed video
signal blockiness indicator 26. Details regarding the method for generating a
video signal blockiness indicator are provided below in reference to Figure 2.
[0015] The
decompressed video signal blockiness indicator 26
produced by the blockiness detection module 18 is transmitted to the
compression module 14 where it may be used to adjust the level of
compression. For example, if the decompressed video signal blockiness
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indicator 26 is above a certain threshold the compression module 14 may
decrease the level of
compression. Similarly, if the decompressed video signal blockiness indicator
26 is below a
certain threshold the compression module 14 may increase the level of
compression. The
decompressed video signal blockiness indicator 26 may also be outputted to a
display module
28 for display to a
user/operator.
[0016]
Referring now to Figure 2, shown therein is a method 200 for determining
the
blockiness of a video signal in accordance with an embodiment. In step 202 a
video signal
comprised of pixels is received by a blockiness detection module. The video
signal may be an
original video signal or a decompressed video signal. In step 204 a region of
interest (ROI) is
selected from the video signal and it is this ROI on which the blockiness
detection is performed.
The ROI is defined by two points: pstart and Pend= The first point, Pstart,
represents the upper left
corner of the ROI and is defined by x and y coordinates )(start and ystart.
The second point, Penn,
represents the bottom right corner of the ROI and is defined by x and y
coordinates xend and
Yend. Referring now to Figure 3, shown therein is a region of interest 302
selected from a video
signal
304.
[0017]
The ROI can be pre-selected or a user can manually select it. If no ROI is
pre-
selected or manually selected then the ROI is defined to be the active viewing
area. Note that
the active viewing area does not include the part of the image that is outside
the normal
rectangular viewing area on a display screen. One of the advantages to
manually selecting an
ROI is that the user may exclude certain areas from being falsely detected as
blocky.
[0018]
In step 206 of method 200, x and y differential values are calculated for
each
pixel in the video signal. To compute the differential values for a pixel of
interest, the pixel of
interest and the nA2-1 pixels in the ROI immediately surrounding the pixel of
interest are
selected to form an nxn filter kernel, where n is any odd integer greater than
one. If, however,
the pixel of interest is on the edge of the ROI such that there are not nA2-1
pixels in the ROI
immediately surrounding the pixel of interest then the nxn filter kernel can
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be formed in a variety of ways. For example, if the ROI is smaller than the
video signal then the pixels outside the ROI can be used to fill the nxn
filter
kernel. Alternatively the pixel itself can be duplicated in the empty spots in
the
filter kernel. In an exemplary embodiment n is equal to 3 such that a 3x3
filter
kernel is selected.
[0019] An NTSC, PAL or SECAM video signal contains two parts that
make up what you see on the display: the intensity part, and the color part.
chroma is the color part and luma is the intensity or brightness part. More
specifically, luma represents the achromatic image without any color, while
the chroma components represent the color information. A blockiness
indicator may be based on both the luma and chroma components, however,
a blockiness indicator based solely on the luma components provides a good
indication of blockiness. Accordingly, to simplify the description of the
blockiness indicator algorithm the remainder of the description will be
described in reference to only the luma values.
[0020] Referring now to Figure 4, shown therein is a 3x3 luma filter
kernel 400 where A5 represents the pixel of interest. The luma filter kernel
400 is represented by nine luma values: A1, A2, A3, A4, A5, A6, A7, Ag and Ag.
Luma values A1, A2 and A3 represent the first row of values from left to
right,
luma values A4, A5 and A6 represent the second row of values from left to
right
and luma values A7, Ag and Ag represent the third row of luma values from left
to right. In one embodiment the luma values will be truncated to 8-bits to
reduce space requirements. When the luma values are truncated to 8-bits the
luma values may range from 0 for black to 255 for white. In an alternative
embodiment the luma values may range from 16 for black to 235 for white.
[0021] X and Y differential values are computed for each 2x2 sub-
block
in the luma filter kernel 400. As shown in Figure 4 there are four 2x2 sub-
blocks of a 3x3 luma filer kernel 400: sub-blocki 402, sub-block2 404, sub-
block3 406 and sub-block4 408. Sub-blocki 402 is comprised of luma values
A1, A2, A4 and A5. Sub-block2 404 is comprised of luma values A2, A3, A5 and
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A6. Sub-block3 406 is comprised of luma values A4, A5, A7 and Ag. Sub-block4
408 is comprised of luma values A5, As, Ag and Ag.
[0022] The x differential value, represented by lx, is equal to the
absolute value of the upper right luma value minus the upper left luma value
plus the lower right luma value minus the lower left luma value, all divided
by
two. For example, for sub-blocki 402 consisting of luma values A1, A2, A4 and
A5, the x differential value is equal to the absolute value of ((A2 ¨ Ai) +.
(A5 ¨
A4)) divided by 2. The x differential value, lx,, for the ith sub-block can be
represented by the following equations:
[0023] /x1 = absolute((A2¨ A1) + (A5¨ A4))/2
[0024] 1)(2= absolute((A3¨ A2) + (A6¨ A5))/2
[0025] 1x3 = absolute((A5¨ 44) + (A8¨ A7))/2
[0026] 1x4 = absolute((A6¨ A5) + (A9¨ A8))/2
[0027] The y differential value, represented by ly, is equal to the
absolute value of the lower left luma value minus the upper left luma value
plus the lower right luma value minus the upper right luma value, all divided
by two. For example, for sub-blocki 402 consisting of luma values A1, AZ A4
and A5, the y differential value is equal to the absolute value of ((A4 ¨ A1)
+
(A5¨ A2)) divided by 2. The y differential value, ly,, for the ith sub-block
can be
represented by the following equations:
[0028] = absolute(04¨ + A2N2
[0029] ly2 = absolute(05¨ A2) + (A6¨ A3))/2
[0030] 1y3 = absolute((A7¨ A4 + (A8¨
[0031] 4/4 = absolute((A8¨ A5) + (As¨ A6))/2
[0032] In step 208 of method 200, qualifier calculations are performed
to determine whether the pixel of interest lies on a horizontal edge in which
a
visible transition occurs (Qh), whether the pixel of interest lies on a
vertical
edge in which a visible transition occur (Qv), and/or whether the pixel is
part
of a flat area (Qf). Horizontal/vertical transitions and flat blocks are key
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indicators of a blockiness problem. This is because horizontal and vertical
transitions increase as the video quality goes from good to poor. However,
horizontal and vertical transitions cannot be relied upon as the sole
indicator
of blockiness since when the video quality decreases from poor to very poor,
the transitions may actually decrease because the blocky blocks may join
together. With respect to flat areas, this is also a good blockiness indicator
since video with severe blockiness tends to have blocks with flat areas.
[0033] Boolean value Qh indicates whether the pixel of interest lies
on
a horizontal edge in which a visible transition occurs. In one exemplary
embodiment the horizontal edge of interest is the one between the first row
and the second row of pixels, but one skilled in the art would be able to
modify
the algorithms so that the horizontal edge of interest is the one between the
second row and the third row of pixels. In this exemplary embodiment the
pixel of interest is said to lie on a horizontal edge in which a visible
transition
occurs if the differential between the first row of luma values and the second
row of luma values meets a minimum edge threshold (Te) and the differential
between all other rows and columns does not reach a maximum non-edge
threshold (Tne). Te represents the minimum differential in luma values that is
visible to the naked eye. In one embodiment Te is equal to 4. In other
embodiments Te ranges from 2 to 15. In one embodiment Tne is equal to 1.
In other embodiments Tne ranges from 1 to 7.
[0034] Generally a pixel is said to lie on a horizontal edge in which
a
visible transition occurs if the x and y differential values for each sub-
block are
above or below Te and Tne. Qh, in general form, can be represented by the
following Boolean expression:
[0035] Qh = (Sbhi AND Sbh2) AND (Sbh3 OR Sbh4
[0036] where Boolean value Sbh; represents the horizontal transition
test for sub-blocki. The specific tests for each sub-block can be represented
by the following Boolean expressions:
[0037] Sbhi = (lxi <= Tne) AND (Iyi >. Te)
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[0038] Sbh2 = (1)(2 <= Tne) AND (Iy2 >= Te)
[0039] Sbh3 = (1x3 <= Tne) AND (Iy3 <= Tne)
[0040] Sbh4 = (1x4 <= Tne) AND (Iy3 <= Tne)
[0041] Boolean value Qv indicates whether the pixel of interest lies
on
a vertical edge in which a visible transition occurs. In one exemplary
embodiment the vertical edge of interest is the one between the first column
and the second column of pixels, but one skilled in the art would be able to
modify the algorithms so that the vertical edge of interest is the one between
the second column and the third column of pixels. In this exemplary
embodiment the pixel of interest is said to lie on a vertical edge in which a
visible transition occurs if the differential between the first column of luma
values and the second column of luma values meets a minimum edge
threshold (Te) and the differential between all other rows and columns does
not reach a maximum non-edge threshold (Tne). Generally a pixel of interest
lies on a vertical edge in which visible transition occurs if the x and y
differential values for each sub-block meet certain thresholds or are below
certain thresholds. Qv, in general form, can be represented by the following
Boolean expression:
[0042] Qv = (Sbvi AND Sbv3) AND (Sbv2 OR Sbv4)
[0043] where the Boolean value Sbv, represents the vertical transition
test for sub-block,. The specific tests for each sub-block can be represented
by the following Boolean expressions:
[0044] Sbvi = <= Tne) AND (Ixi >= Te)
[0045] Sbv2 = (Iy2 <= Tne) AND (Ix2 <= Tne)
[0046] Sbv3 = (1y3 <= Tne) AND (1x3 >= Te)
[0047] Sbv4 = (ly4 <= Tne) AND (Ix3 <= Tne)
[0048] The Boolean value Qf indicates whether the pixel of interest
is
part of a flat area. A flat area can be described as one in which all of the
luma
values in luma filter kernel 400 are the same. Accordingly, a pixel lies in a
flat
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area if all x and y differential values are equal to zero. Qf can be
represented
by the following Boolean expression:
[0049] Qf = ((lyi=0) AND (1x1=0)) AND ((ly2=0) AND (1x2=0)) AND
((1y3=0) AND (1x3=0)) AND ((1y4=0) AND (1x4=0))
[0050] In step 210 of method 200, the horizontal border transitions
count (bh), the vertical border transitions count (by), the flat area count
(bf)
and the macroblock flat area count (bc) for the ROI are calculated. Step 210
may be performed in hardware by, for example, a field programmable gate
array (FPGA) or in software.
[0051] The horizontal border transitions count (bh) represents the total
number of pixels of the ROI that are part of a visible horizontal transition
along
a block border. Since DCT codes a video signal in 8x8 block, a visible
horizontal transition along a block border is a horizonal transition that
lasts a
block (8 pixels) or multiple blocks (n*8 pixels). Accordingly, a transition
that
only lasts 5 pixels will not be considered a visible horizontal transition
along a
block border. In one embodiment the horizontal border transitions count (bh)
is calculated by horizontally counting the number of valid horizontal
transitions
(Qh) in a row and if the count plus 2 is divisible by 8 then the horizontal
border
count (bh) is increased by the number of valid horizontal transitions plus 2.
With respect to the 'plus 2' aspect of the calculation, a pixel of interest is
only
said to be on a valid horizontal transition if there is a valid differential
between
(1) the pixel of interest and the one above it, (2) the pixel to the left of
the pixel
of interest and the one above that one, and (3) the pixel to the right of the
pixel of interest and the pixel above that one. Accordingly the pixels on the
end of a horizontal transition will not qualify as being on a visible
horizontal
transition along a block border because either the pixels to the left or the
right
will not meet the test. Therefore the 'plus 2' is required to get an accurate
count of pixels in horizontal border transitions.
[0052] Similarly, the vertical border transitions count (by)
represents the
total number of pixels of the ROI that are part of a visible vertical
transition
along a block border. In one embodiment the number of vertical border
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transitions count (by) is calculated by vertically counting the number of
valid
vertical transitions (Qv) in a row and if the count plus 2 is divisible by 8
then
the vertical border count (by) is increased by the number of valid horizontal
transitions plus 2. The reason for the 'plus 2' is described above in relation
to
the horizontal block transition count (bh). In an alternative embodiment, the
vertical border transitions count (by) is calculated by counting the number of
horizontal pixels that are between two pixels on vertical transitions (Qv),
and if
the number plus 1 is divisible by 8, then the vertical border transitions
count
(by) is increased by 1. This alternative embodiment would be used when the
counting is done in hardware that reads in the pixels on a row basis instead
of
a column basis, and it would be too costly to transpose the image in
hardware.
[0053] The flat area count (ba) represents the total number of pixels
in
the ROI that are in a flat area. This is simply calculated by adding up the
number of Qfs in the ROI.
[0054] The macroblock flat area count (bc) represents the total
number
of pixels in the ROI that are in a flat macroblock. A flat macroblock is a
flat
area constituting one or more 8x8 blocks. In one embodiment the number of
macroblock flat areas is calculated by horizontally counting the number of
pixels in flat areas (Qf) in a row and if the count plus 2 is divisible by 8
then
the macroblock flat areas count is increased by the count plus 2. The reason
for the 'plus 2' is described above in relation to the horizontal block
transition
count (bh).
[0055] Pseudo-code describing the process of calculating the
horizontal
border transitions count (bh), the vertical border transitions count (by),
flat
areas count (bf), and the macroblock flat areas count (bc), is shown below:
[0056] Counting transitions and flat areas
V = yend ¨ ystart + 1;
H = xend ¨ xstart + 1;
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bh = 0;
by = 0;
bf = 0;
bc =0;
for y=ystart:yend
cnth = 0;
cntv = 0;
cntc = 0;
for x=xstart:xend
if qh(x,y)
cnth = cnth + 1;
else
t = cnth + 2;
if mod (t, 8) == 0
bh = bh +1
end
cnth = 0;
end
if ¨qv(x,y)
cntv = cntv + 1;
else
if mod (cntv + 1,8)
by = by + 1;
end
cntv = 0;
end
if qf(x,y)
bf= bf +1;
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end
if qf(x,y)
cntc = cntc + 1;
else
t = cntc + 2;
if mod (t, 8) == 0
bc = bc + t;
end
cntc = 0;
end
end
end
[0057] In step 212 of method 200, at least one blockiness indicator
is
generated. A blockiness indicator is a numerical value that indicates the
level
of blockiness in the video signal. In one embodiment three blockiness
indicators are generated: a block border indicator (bbi), a flat area
indicator
(fai) and a flat block indicator (fbi). The block border indicator (bbi) is
equal to
the number of pixels along visible macroblock borders multiplied by 10 and
then divided by the total number of pixels in the ROI. The number of pixels
along visible macroblock borders is multiplied by 10 to bring the block border
indicator (bbi) on a comparable scale as the flat area indicator (fai) and a
flat
block indicator (fbi). There is required because bbi is a measurement of
borders whereas fai and fbi are measurements of areas so naturally fai and fbi
will be significantly larger than bbi.
[0058] The number of pixels along visible macroblock borders is equal
to the horizontal border transitions count (bh) plus the vertical border
transitions count (by) calculated in step 210. The total number of pixels in
the
ROI (N) is equal to the width of the ROI multiplied by the height of the ROI.
N
can be represented by the following formula:
[0059] N = (Xend Xstart + * (Yend Ystart +
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[0060] The block border indicator (bbi) can be thus represented by
the
following formula:
[0061] bbi = 10 * (bh + bv) / N
[0062] The block border indicator (bbi) provides a direct correlation
to
the blockiness of an image. For example, a bbi > 0.1 indicates a high level of
blockiness. For most clean pictures bbi is close to 0.
[0063] The flat area indicator (fai) is equal to the total number of
pixels
in flat areas (bf) divided by the total pixels in the ROI. The flat area
indicator
(fai) can be represented by the following formula:
[0064] fat = bf / N
[0065] The flat area indicator (fai) is useful in distinguishing
graphics
from video. For example, a fai > 0.6 indicates the image is likely graphics,
not
video.
[0066] The flat block indicator (fbi) is equal to the number of
pixels in
flat macroblocks (bc) divided by the total pixels in the ROI. The flat block
indicator (fbi) can be represented by the following formula:
[0067] fbi = bc / N
[0068] The flat block indicator (fbi) is useful in detecting picture
breakup. For example, if fbi >0.1 then the video is likely corrupted.
[0069] In step 214 of method 200, recursive filtering is performed on
the blockiness indicator(s). Step 214 may be performed by software residing
on a central processing unit (CPU). The output of the filter, blockiness(n),
is
equal to the difference between the current blockiness indicator and the
previous filter output multiplied by a factor, k, plus the previous filter
output.
The filter output, blockiness(n) can be represented by the following formula:
[0070] blockiness(n) = (blockiness indicator - blockiness(n-1))*k +
blockiness(n-1)
[0071] Factor k is equal to a user determined value, Kuser, when the
difference between the blockiness indicator and the previous filter output
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(blockiness(n-1)) is less than an adjustable threshold, Tchange, otherwise k
is
equal to zero. Kuser is the value that determines how smooth the filter output
is. The larger Kuser is the more the filter output is affected by changes in
the
blockiness indicator. Conversely, the smaller Kuser is, the less the filter
output is affected by changes in the blockiness indicator and the smoother it
is. In one embodiment Kuser can be any value of 10" where n is any 0.1
increment between -4 and 0.
[0072] Tchange is an adjustable value that controls whether large
changes in the blockiness indicator are immediately reflected in the filter
output. The larger Tchange is, the larger the difference between the
blockiness indicator and the previous filter output (blockiness(n-1)) has to
be
before it will be ignored by the filter.
[0073] The recursive filtering of step 214 may be performed on any or
all of the blockiness indicators such as the block border indicator (bbi), the
flat
area indicator (fai), and the flat block indicator (fbi). When the block
border
indicator (bbi) is filtered, the filtered output is referred to as bb. When
the flat
area indicator (fai) is filtered, the filtered output is referred to as fa.
Similarly,
when the flat block indicator (fbi) is filtered, the filtered output is
referred to as
fb. The recursive filtering may be performed by an IIR filter.
[0074] In step 216 of method 200, thresholding is performed on the
filter outputs (i.e. bb, fa and fb) to determine if a macroblock alert should
be
produced. Step 216 may be performed by software residing on a central
processing unit (CPU). A macroblock alert is produced if certain filter
outputs
reach a predetermined threshold. The specific filter outputs that would
trigger
an alert and the specific thresholds required to trigger an alert are
configurable.
[0075] In one embodiment a macroblock alert is triggered when the bb,
fa and fb outputs each reach a specific threshold. The threshold that relates
to the bb output is referred to as Tbb. The threshold that relates to the fa
output is referred to as Tfa. Similarly the threshold that relates to the fb
output
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is referred to as Tfb. The macroblock alert output can be represented by the
following Boolean
expression:
[0076] macroblock alert = (bb > Tbb) AND (fa < Tfa) AND (fb > Tfb)
[0077] Referring now to Figure 5, shown therein is a block diagram of
an alternate
embodiment of a blockiness detection system 500. The blockiness detection
system 500
includes a video source 512, a compression module 514, a decompression module
516, a first
blockiness detection module 518 and a second blockiness detection module 530.
The video
source 512 produces a video signal 520. Compression module 514 compresses the
video
signal 520 and outputs a compressed video signal 522. Decompression module 516
decompresses the compressed video signal 522 and outputs a decompressed video
signal 524.
The first blockiness detection module 518 analyzes the decompressed video
signal 524 and
generates a decompressed video signal blockiness indicator 526. The
decompressed video
signal blockiness indicator 526 is then transmitted to the compression module
514.
[0078] In parallel the second blockiness detection module 530
receives the video signal
520 generated by the video source 512 and generates a video signal blockiness
indicator 532.
The compression module 514 compares the video signal blockiness indicator 532
to the
decompressed video signal blockiness indicator 526 and if the decompressed
video signal
blockiness indicator 526 is higher than the video signal blockiness indicator
532 by a
predetermined amount then the compression is too high and the compression
module 514
readjusts the compression.
[0079] Although preferred embodiments of the invention have been
disclosed for
illustrative purposes, those skilled in the art will appreciate that many
additions, modifications,
and substitutions are possible and that the scope of the claims should not be
limited by the
embodiments set forth herein, but should be given the broadest interpretation
consistent with
the description as a whole.
